aslide9 receivers 1
TRANSCRIPT
-
7/29/2019 Aslide9 Receivers 1
1/14
1
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
Chelmsford Amateur Radio Society
Advanced Course(4) Receivers
Part-1 Receiver Parameters
-
7/29/2019 Aslide9 Receivers 1
2/14
2
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
Receiver Parameters
Important performance measures for receivers
Frequency stability
Selectivity
Bandwidth
Sensitivity
Dynamic range
Signal-to-noise ratio (SNR)
Effect of RF amplifiers & pre-amps
Transverters
-
7/29/2019 Aslide9 Receivers 1
3/14
3
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
Frequency Stability
Frequency stability is the same as for transmitters
Accuracy of tuning to an entered or displayed frequency
Ability to remain on frequency without drifting off
Often given in ppm parts-per-million
1ppm error at 28MHz is 28Hz.
-
7/29/2019 Aslide9 Receivers 1
4/14
4
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
Selectivity
Selectivity is the ability to separate the wanted signal from nearby
unwanted signals (other stations)
Frequency MHz
Amplitude
Wanted signal
(weak)
Unwanted signal
(strong) Filter response
-
7/29/2019 Aslide9 Receivers 1
5/14
5
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
Selectivity Measures
Measures of selectivity
60-dB bandwidth
adjacent channel rejection ratio (VHF, UHF channelised)
but how far away is the next channel (12.5kHz?)
For SSB, may specify opposite sideband rejection
Frequency MHz
Amplitu
de
0dB -Filter response
-60dB - 60dB bandwidth
-
7/29/2019 Aslide9 Receivers 1
6/14
6
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
Bandwidth
Band of frequencies which the receiver should accept
Eg. CW (A1A) morse typically 300Hz
SSB uses 2.5 to 3kHz
VHF FM typically 7.5 or 15kHz
Usually 3dB BW specified, but not always!
Frequency MHz
Amplitude
0dB -Filter response
-3dB - 3dB bandwidth
-
7/29/2019 Aslide9 Receivers 1
7/14
7
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
Sensitivity
Sensitivity defines the limit of detection of weak signals.
Receivers must have enough gain to bring weakest signals to
comfortable level.
the gain does not define sensitivity
Sensitivity is determined by 2 factors:
Bandwidth of the receiver
the wider the bandwidth, the more noise power it lets in
Noise figure of the receiver front-end
a noisy receiver needs more signal to overcome the noise
Receivers bandwidth should match the transmitted bandwidth.
so as not to exclude any signal or accept unnecessary noise.
-
7/29/2019 Aslide9 Receivers 1
8/14
8
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
Sensitivity Definitions
Sensitivity is defined as the receiver input signal level for a given
SINAD at the output
eg. 0.2V for 12dB SINAD
SNR is Signal-to-noise ratio
SINAD is Signal + Noise + Distortion
Intelligible speech needs about 12dB SINAD
PowerNoise
PowerSignalSNR
DistortionNoiseDistortionNoiseSignalSINAD
Usually expressed in dB
-
7/29/2019 Aslide9 Receivers 1
9/14
9
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
Dynamic Range
Dynamic range is the range of signal levels between the smallest
and greatest a receiver can handle
Lower limit set by sensitivity
Upper limit set by distortion or AGC control range
In practice, we are more concerned about dynamic range to
handle unwanted out-of-band signals (AGC doesnt apply).
How large an unwanted signal will it reject without affecting
sensitivity to wanted signals
Overload level may be specified for receiver front-end (RF
amplifier, mixer) as the 1dB compression point
-
7/29/2019 Aslide9 Receivers 1
10/14
10
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
Dynamic Range
1dB compression point
Power level where amplifier gain drops by 1dB
Input level
Outputlevel
1dB
Amplifier linearity curve
Input compression
point
Output compression
point
Noise floor
-
7/29/2019 Aslide9 Receivers 1
11/14
11
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
Noise Figures
There is a limit of physics to receiver sensitivity
Even for perfect receivers that add no noise (0dB NF)
Real receivers can get within a few dB of the limit
Typical noise figures
HF receiver; 12 to 20dB - not as critical as atmospherics dominate
VHF receiver; 6 dB
Microwave receiver; 2dB
Raw sensitivity is traded for dynamic range and selectivity in
environments where these are more important
Adjusting RF gain can optimise sensitivity vs. dynamic range
Expressed in dB
outputatSNRinputatSNRNF
-
7/29/2019 Aslide9 Receivers 1
12/14
12
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
RF Amplifiers
If the RF pre-amp has a similar noise figure to the receiver
Sensitivity not improved, dynamic range made worse
If the RF pre-amp has a better noise figure to the receiver
Sensitivity improved, dynamic range still worse
If there is a feeder loss before the receiver (masthead amp)
Without preamp, receiver performance degraded by loss
Amplifier can overcome feeder loss, performance improved
RF pre-amplifier Receiver
-
7/29/2019 Aslide9 Receivers 1
13/14
13
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
Downconverters
Downconverters convert signals from one band to another.
This enables reception of signals out of the tuning range of the
receiver.
Use of downconverters common for VLF, microwave, and weathersatellite.
Operator must mentally add the frequency offset to the frequency
displayed by the receiver.
Down-
converterReceiver
-
7/29/2019 Aslide9 Receivers 1
14/14
14
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
Transverters
Transverters are bi-directional frequency converters
May be used for multimode (SSB J3E , CW A1A) operation by
using a HF transceiver on VHF or UHF
Displayed frequency must be mentally adjusted
Reliable high isolation switching is vital
Down-
converter
Up-
converterTrans-
ceiver
Power
amp